Mood Stabilizers & Bipolar Disorder: Lithium & Beyond
One of psychiatry's most powerful drugs is also one of its simplest — a single element, lithium, plucked from the periodic table. It can calm mania and, remarkably, cut the risk of suicide. But it works in a window so narrow that a hot summer's day, a new painkiller, or a water pill can tip a stable patient into poisoning. Lithium is the ultimate lesson in a drug where the dose IS the danger.
A woman with bipolar disorder has been stable on lithium for years. Then a heatwave hits; she sweats heavily, drinks too little, and starts taking ibuprofen for a headache. Within days she's confused, her hands shaking coarsely, unsteady on her feet — lithium toxicity. Her dose never changed. What changed was her body's ability to clear the drug: dehydration and the painkiller quietly pushed her lithium level from therapeutic into toxic. This is the defining feature of lithium, and it comes straight from the pharmacokinetics we've already learned.
What a mood stabilizer does
Bipolar disorder swings between mania and depression. Bipolar disorder is defined by episodes of mania (elevated, energized, impulsive) alternating with depression. A mood stabilizer is a drug that dampens these swings and prevents both poles from returning. Lithium is the classic — first-line, effective for acute mania and for long-term prevention, and uniquely shown to reduce suicide risk. Exactly how it works is still not fully understood (it likely acts on intracellular signalling pathways rather than a single receptor), which is a reminder that a drug can be enormously useful even when its precise mechanism remains a puzzle.
The narrow window — and why the body matters
Lithium's defining problem is its narrow therapeutic index: the effective level and the toxic level sit dangerously close, so it needs regular blood-level monitoring (a textbook use of therapeutic drug monitoring). Early toxicity brings tremor, nausea, and diarrhoea; worse toxicity brings confusion, unsteadiness (ataxia), seizures, and coma. Crucially, lithium is cleared entirely by the KIDNEY and handled like sodium — so anything that makes the body hold onto sodium and water raises lithium. Dehydration, a low-salt state, and three drug classes in particular — NSAIDs, thiazide diuretics, and ACE inhibitors — all reduce lithium clearance and can tip a stable patient into toxicity. That's exactly what happened in the opening case.
Long-term lithium also affects two organs to watch: the thyroid (causing hypothyroidism) and the kidney (causing a form of diabetes insipidus, with excessive thirst and urination). And it's teratogenic — linked to a heart defect (Ebstein's anomaly) — so it's used cautiously in pregnancy. Regular checks of lithium level, thyroid, and kidney function are part of the deal.
- Mood stabilizers prevent both mania and depression in bipolar disorder.
- Lithium is first-line, treats mania, and uniquely reduces suicide risk.
- Narrow therapeutic index → needs blood-level monitoring; toxicity: tremor → ataxia, confusion, seizures.
- Renally cleared (like sodium): dehydration, NSAIDs, thiazides & ACE inhibitors raise its level.
- Long-term risks: hypothyroidism, nephrogenic diabetes insipidus; teratogenic.
Beyond lithium
Lithium isn't the only option. Several antiepileptics double as mood stabilizers: valproate is effective for acute mania (but teratogenic, so avoided in women of childbearing potential), carbamazepine is an alternative, and lamotrigine is particularly good at preventing the DEPRESSIVE pole. The atypical antipsychotics (quetiapine, olanzapine, risperidone, aripiprazole) are now widely used for acute mania and maintenance too — a bridge to the next article. One critical caution ties this series together: giving an ANTIDEPRESSANT alone to a bipolar patient can flip them into mania, so antidepressants are used carefully and usually alongside a mood stabilizer.
Lithium toxicity is usually not about a wrong prescription — it's about a change in the body. Since lithium follows sodium and leaves only through the kidney, ask before starting any new drug or during any illness: has anything reduced this patient's fluid or kidney clearance? A stomach bug, a heatwave, a new diuretic or NSAID, or an ACE inhibitor can all silently raise the level. The dose on the bottle didn't change — the patient did.
- Not monitoring lithium levels. Its narrow window makes regular blood tests essential.
- Adding an NSAID, thiazide, or ACE inhibitor without rechecking lithium. They raise its level.
- Ignoring dehydration (heat, vomiting) in a lithium patient — a common toxicity trigger.
- Giving an antidepressant alone in bipolar disorder. It can precipitate mania.
A stable lithium patient becomes toxic after a heatwave and starting ibuprofen. Why?
- Mood stabilizers prevent both poles of bipolar disorder; lithium is first-line and cuts suicide risk.
- Lithium has a narrow window — monitor levels; it's renally cleared like sodium.
- Dehydration, NSAIDs, thiazides & ACE inhibitors raise lithium and cause toxicity.
- Alternatives: valproate, carbamazepine, lamotrigine (depressive pole), atypical antipsychotics.
- Katzung BG. Basic & Clinical Pharmacology — Drugs Used in Bipolar Disorder (lithium & anticonvulsants).
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Lithium & mood stabilizers.
- Rang HP, Dale MM, et al. Rang & Dale's Pharmacology — Drugs used to treat bipolar disorder.
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Lithium & mood-stabilizing drugs.
- Clinical guidelines — Lithium monitoring, toxicity & interactions (NSAIDs, thiazides, ACE inhibitors).
